The present invention relates generally to radio frequency systems and, more particularly, to a radio frequency system for detecting resonant tags and for ascertaining information stored in the tags.
The use of radio frequency systems for detecting and preventing theft or unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, such security systems, known generally as electronic article security (EAS) systems employ a tag which is associated with or which is secured to the article to be protected. Tags may take on many different sizes, shapes and forms depending upon the particular type of EAS system in use, the type and size of the article, its packaging, etc. In general, such EAS systems are employed for detecting the presence of a tag as the protected article passes through or near a surveilled security area or zone. In most cases, the surveilled security area is located at or near an exit or entrance to the retail establishment or other facility.
One such electronic article security system which has gained widespread popularity utilizes a tag which includes a resonant circuit which, when interrogated by an electromagnetic field having prescribed characteristics, resonates at a single predetermined detection frequency. When an article having an attached resonant tag moves into or otherwise passes through the surveilled area, the tag is exposed to an electromagnetic field created by the security system. Upon being exposed to the electromagnetic field, a current is induced in the tag creating an electromagnetic field which changes the electromagnetic field created within the surveilled area. The magnitude and phase of the current induced in the tag is a function of the proximity of the tag to the security system, the frequency of the applied electromagnetic field, the resonant frequency of the tag, and the Q factor of the tag. The resulting change in the electromagnetic field created within the surveilled area because of the presence of the resonating tag can be detected by the security system. Thereafter, the EAS system applies certain predetermined selection criteria to the signature of the detected signal to determine whether the change in the electromagnetic field within the surveilled area resulted from the presence of a tag or resulted from some other source. If the security system determines that the change in the electromagnetic field is the result of the presence of a resonant tag, it activates an alarm to alert appropriate security or other personnel.
While electronic article security systems of the type described above function very effectively, a limitation of the performance of such systems relates to false alarms. False alarms occur when the electromagnetic field created within the surveilled area is disturbed or changed by a source other than a resonant tag and the security system, after applying the predetermined detection criteria, still concludes that a resonant tag is present within the surveilled area and activates an alarm, when in fact no resonant tag is actually present. Over the years, such EAS systems have become quite sophisticated in the application of multiple selection criteria for resonant tag identification and in the application of statistical tests in the selection criteria applied to a suspected resonant tag signal. However, the number of false alarms is still undesirably high in some applications. Accordingly, there is a need for a resonant tag for use in such electronic article security systems which provides more information than is provided by present resonant tags in order to assist such electronic article security systems in distinguishing signals resulting from the presence of a resonant tag within a surveilled area and similar or related signals which result from other sources.
One method of providing additional information to the EAS system is to provide a tag which responds to the interrogation signal with a signal at a different frequency than the frequency of the interrogation signal or at more than one frequency. Heretofore, single tags having one of these properties required that the tag include an active element such as a transistor, or a non-linear element, such as a rectifier or diode, both of which elements negate manufacturing the tag as a planar passive device using the technology in place for manufacturing such resonant tags.
Another method of providing additional information to the EAS system is to have two or more resonant tags, each with a different resonant frequency, secured to the article being protected. For example, the resonant frequency of a second tag could be offset from the resonant frequency of a first tag by a known amount. In this manner, the simultaneous detection of two or more signals at specific predetermined separated frequencies each having the characteristics of a resonant tag signal would have a high probability of indicating the presence of the multiple resonant tags in the surveilled area since the probability of some other source or sources simultaneously generating each of the multiple signals at each of the predetermined frequencies is very small.
The concept of utilizing a plurality of tags resonant at different frequencies on each article has not been generally accepted because of the requirement for physically separating the tags by a substantial distance in order to preclude the tags from interacting in such a way that the respective resonant frequencies are altered in an unpredictable way. Placing the resonant tags at a substantial distance from each other is disadvantageous because at best it requires separate tagging operations thereby substantially increasing the cost of applying the resonant tags. In addition, some articles are just not large enough to permit the two or more tags to be separated enough to preclude interaction. Separating the tags by a significant distance also affects the orientation and, therefore, the signal strength from the tags thereby limiting detectability of one or more of the tags.
There are also radio frequency systems, known generally as radio frequency identification (RFID) systems, which operate with resonant tags for identifying articles to which the resonant tag is attached or the destination to which the articles should be directed. The use of resonant circuit tagging for article identification is advantageous compared to optical bar coding in that it is not subject to problems such as obscuring dirt and may not require exact alignment of the tag with the tag detection system. Generally, the resonant tags used in RFID systems store information about the article by activating (or deactivating) the resonant circuit patterns which have been printed, etched or otherwise affixed to the tag. Typically, systems utilizing multiple tuned circuit detection sequentially interrogate each resonant circuit with a signal having a frequency of the resonant circuit and then wait for reradiated energy from each of the tuned circuits to be detected. The result of having to sequentially interrogate the tag at each of the different frequencies is a slow detection system that limits the speed at which the articles may be handled.
The present invention employs a tag having a plurality of resonant circuits, each of which are electromagnetically coupled to a receiving resonant circuit. Upon interrogation by a pulse at the receiving frequency, the tag radiates a detectable electromagnetic signal having frequency components which correspond to the resonant frequencies of the resonant circuits. Accordingly, the present invention is capable of reducing the false alarm rate in EAS applications without the need for separate tags with distinct frequencies being placed on an article; and also, is capable of providing information stored on the tag in RFID applications.